The present invention relates to a method and apparatus for producing white cement clinker.
In production of white cement clinker, not only the strength of a final product but also a higher degree of whiteness thereof are demanded. In order to improve the whiteness, apparently raw materials should contain as less iron and manganese as possible; moreover, it is necessary to rapidly cool clinker after clinker is burned in a reducing atmosphere or is first burned in an oxidizing atmosphere and then is subjected to a reduction process by spraying a fuel such as heavy oil. It is well known in the art that rapid cooling of clinker to temperatures below 700.degree. C. must be effected under the condition that the clinker is prevented from contacting with the air; otherwise the clinker would be re-oxidized by oxygen contained in the air.
So far rotary kilns have been used to produce white cement clinker. Clinker discharged from a rotary kiln is immersed in water so as to cool clinker while preventing the same from contacting with the air. Since clinker burned in the rotary kiln has a wide range of particle sizes, clinker cannot be cooled uniformly. If clinker is coarse particle size is immersed in water for a long time to be cooled to temperatures lower than 700.degree. C., much water is needed and clinker in fine particle size is overcooled. Furthermore the overall temperature of clinker drops too low so that the sensible heat of clinker cannot be used to preheat the combustion air. Moreover, even when clinker has been recovered from the water, the water adheres to clinker so that there arises a problem that the strength of cement is degraded. In addition, a large quantity of steam is produced due the heat exchange between the water and high-temperature clinker and enters the rotary kiln so that the heat consumption of the rotary kiln is increased. On the other hand, when the quantity of water is decreased, clinker in fine particle size is satisfactorily cooled but clinker in coase particle size is not satisfactorily cooled so that when clinker is made into contact with the air, it is oxidized again and consequently its whiteness is degraded. In order to overcome the above-described problems, clinker must be so burned as to have a uniform particle size, but it is impossible to do so when clinker is burned in the rotary kiln. According to a prior art known from Japanese Patent Application laid open under No. 26057/1983, clinker in fine particle size is discharged from a kiln through an annular discharge gap located at a position spaced apart from the discharge end of the kiln by a distance substantially equal to the radius of the kiln and is directed to fall upon an upper part of a conveyor inclined at an angle in a water camber. Clinker in coarse particle size from the discharged end of the kiln is sorted or classified by a sorting or classifying screen such that the coarser the particle size of clinker, the further below clinker is dropped on the conveyor. In other words, the period of time during which clinker in fine particle size is immersed in the water is shorter while the coarser the particle size of clinker, the longer clinker is immersed in the water so that clinker is uniformly cooled. However, the clinker particles cannot be satisfactorily sorted or classified by the discharge gap and the sorting or classifying screen so that there are left much to be improved.
The present invention has for its object to overcome the problem that the prior art methods or processes cannot uniformly cool clinker because burned clinker has a wide range of particle sizes. The present invention also overcomes the problems that the heat consumption of the kilns is increased and that the whiteness is degraded. That it, according to the present invention, relatively fine clinker particles having a narrow particle-size-distribution range can be obtained by burning. Thus, the primary object of the present invention is to provide a method and apparatus for producing white cement clinker which can completely overcome the above and other problems encountered in the prior art methods and apparatus.